The present invention relates to a terrestrial flight telecommunications system and a method of operating same, and more particularly it relates to synchronising an air station with a ground station.
Terrestrial flight telephone systems are being developed for commercial airliners to enable passengers on those airliners to communicate with the public switched telephone network (PSTN). These systems, some of which are already in service on a trial basis, operate in a manner similar to a cellular mobile telephone system, in that each aircraft comprises what is termed an "air station" which communicates with a selected one of a plurality of ground stations, one way in which the selection process could be made being described in co-pending U.S. patent application Ser. No. 08/688,159, filed Jul. 29,1996, entitled "A Telecomununications System". An example of such a terrestrial flight telecommunications system is found in the European Telecommunications Standard Institute (ETSI) entided "Radio Equipment and Systems (RES) Terrestrial Flight Telephone System (TFTS) Part 2: Search Services, Radio Interface" published as prETS300 326-2, the contents of which are hereby incorporated by way of reference.
A complete description of a terrestrial flight telephone system is beyond the scope of this specification and the reader is referred to the above cited ETSI document for a detailed description of such a system. This invention is concerned with one particular aspect of such a system, namely, maintaining synchronisation between an air station aboard at aircraft and a ground station. The reason why this synchronisation is necessary arises as a result of it being necessary to have a time division multiple access (TDMA) system which permits a number of air stations to simultaneously use a particular ground station on a common frequency. This is achieved by the ground station defining sequential frames, each frame contining a number of time slots, one or more of those time slots being assigned to each respective air station. This in turn requires very accurate control of the transmission of signals from the air stations to ensure that they arrive in the appropriate time slot, for any drift in one transmission time will interfere with signals associated with an adjacent slot.
The above referred to ETSI document discloses in detail one way in which a number of air stations can be synchronize to a common ground station, and as an understanding of this is necessary in order to appreciate the present invention, a summary of this will now be provided with reference to FIG. 1.
Referring to FIG. 1 there is illustrated a schematic timing diagram which enables synchronisation to be achieved between a ground station and a number of air stations in a time division multiple access (TDMA) terrestrial flight telephone system (TFTS). The ground station (GS) has a clock signal common to a number of ground stations and the clock signal defines network time to which everything is ideally synchronize. In each frame defined by the ground station the ground station transmits a synchronisation signal. This is received by the receivers of air stations (AS) on an aircraft within the cell as illustrated in FIG. 1. The time of receipt will depend on the Lime propagation ground to air (TPGA) determined by the distance between the ground station and the respective air station.
Consider now air station 1 having a receiver RX. Immediately on receiving the digital sync word from the ground station air station 1 itself transmits a sync word which is received by the ground station at a time T equal to twice TPGA. The ground station determines the value of TPGA and transmits this information to the air station by a radio control channel (RCCH) which is comprised in one slot in each frame.
Air station 1 receives from the ground station the TPGA and a slot assigned to it in which the ground station will receive subsequent transmissions from that air station. The air station then transmits data (TX.sub.1) to be received in that slot, advancing the transmit slot by a time TPGA such that the data arrives in synchronism with the assigned receive slot at the ground station.
The air station comprises an on-board oscillator and having received both the TPGA and a synchronisation signal from the ground station uses the oscillator to generate an image of network time in the air station. Of course, as already discussed the synchronisation signal does not arrive synchronize with the network clock but differs by a time equal to TPGA. TPGA in turn varies with the distance of the aircraft from the ground station. However, having known the TPGA at initialisation (he air station is able to track TPGA by comparing the time of arrival of subsequent synchronisation signals with the image network time generated by the oscillator. The updated TPGA is used to control the time of subsequent transmissions so that they arrive at the ground station in their allocated slots of subsequent frames.
To maximise traffic density the "dead time" between slots needs to be minimised, but this in turn is dictated by the ability of all air stations and ground stations to be in absolute synchronisation. Absolute synchronisation does not exist due to various factors such as noise, and more importantly drift in the air station oscillators and therefore drift in the air station's image of network time. Drift in the ground station is not a problem as the ground station can be synchronised to network time by a global positioning system or some other means. Also ground stations can have more accurate oscillators being located in a less hostile environment, and also costs of the ground station oscillators is of less concern due to the limited number compared to the potential number of air stations.
The system compensates for "short term" oscillator drift in the air stations, (that is drift which occurs during the period the call is to be transmitted between a ground station and an air station or vice versa), by monitoring at the ground station the time at which transmissions from a particular air station are received relative to the allocated slot. Then at a set time period, for example every ten seconds, the signal from the ground station to the air station is substituted with a RCCR correction signal which advises whether or not the air station needs to advance or retard its transmissions to maintain synchronisation with the ground station. This known system works so long as the oscillator frequency is within a predetermined range. If the oscillator frequency moves outside that range the air station will be so far out of synchronisation by the time it receives a correction signal that it may not be able to receive the correction signal, or the correction applied may not be able to keep up with the rate of drift. This will not only cause the transmission between the ground station and that air station to fail but it may also interfere with transmissions by other air stations using that ground station and could cause these to fail also.
The drift rate of the oscillator can be calibrated prior to installation, but the "long term" drift rate may exceed acceptable limits after a period in service, for example ten years. This long term drift can be addressed by servicing the air station at regular intervals, having a service engineer check the oscillator drift rate and trim the oscillator if necessary. Alternatively it may be possible to obtain oscillators which are sufficiently stable for the 10 year life expectancy of an air station. Such oscillators have not yet been identified by the inventor the problem being that more accurate oscillators use larger crystals, and in an aircraft environment the larger crystal will be subjected to more stress than a smaller crystal resulting in further inaccuracies. More accurate oscillators are also more expensive.